Ans: B
$\begin{array}{cl}
& \dfrac{6}{n-6} – \dfrac{7}{n-7} \\
= & \dfrac{6(n-7)}{(n-6)(n-7)} -\dfrac{7(n-6)}{(n-6)(n-7)} \\
= & \dfrac{6(n-7)-7(n-6)}{(n-6)(n-7)} \\
= & \dfrac{6n-42 -7n +42}{(n-6)(n-7)} \\
= & \dfrac{-n}{(n-6)(n-7)} \\
= & \dfrac{n}{(n-6)[ -(n-7)]} \\
= & \dfrac{n}{(n-6)(7-n)}
\end{array}$
